The intervertebral discs, which are located between adjacent vertebrae in the spine, provide structural support for the spine as well as the distribution of forces exerted on the spinal column. An intervertebral disc consists of three major components: cartilage endplates, nucleus pulpous, and annulus fibrosus. The central portion, nucleus pulpous, is relatively soft and gelatinous, being composed of about 70 to 90% water. It has a high proteoglycan content and contains a significant amount of Type II collagen and chondrocytes. Surrounding the nucleus is the annulus fibrosus, which has a more rigid consistency and contains an organized fibrous network of approximately 40% Type I collagen, 60% Type II collagen, and fibroblasts. The annular portion serves to provide peripheral mechanical support to the disc, afford torsional resistance, and contain the softer nuclear portion while resisting its hydrostatic pressure.
Intervertebral discs, however, are susceptible to a number of injuries. Disc herniation occurs when the nucleus begins to extrude through an opening in the annulus, often to the extent that the herniated material impinges on nerve roots in the spine. The posterior and posterio-lateral portions of the annulus are most susceptible to attenuation or herniation, and therefore, are more vulnerable to hydrostatic pressures exerted by vertical compressive forces on the intervertebral disc. Various injuries and deterioration of the intervertebral disc and annulus fibrosus are discussed by Osti et al., Annular Tears and Disc Degeneration in the Lumbar Spine, J. Bone and Joint Surgery, 74-B(5), (1982) pp.678–682; Osti et al., Annulus Tears and Intervertebral Disc Degeneration, Spine, 15(8) (1990) pp. 762–767; Kamblin et al., Development of Degenerative Spondylosis of the Lumbar Spine after Partial Discectomy, Spine, 20(5) (1995) pp. 599–607.
Many treatments for intervertebral disc injury have involved the use of nuclear prostheses or disc spacers. A variety of prosthetic nuclear implants are known in the art. For example, see Bao et al., U.S. Pat. No. 5,047,055, which teaches a swellable hydrogel prosthetic nucleus. Other devices known in the art, such as intervertebral spacers, use wedges between vertebrae to reduce the pressure exerted on the disc by the spine. Intervertebral disc implants for spinal fusion are known in the art as well, as taught by Brantigan, U.S. Pat. Nos. 5,425,772 and 4,834,757.
Yet other approaches are directed toward replacement of the total disc, e.g., using a cage in the manner provided by Sulzer. Its BAK® Interbody Fusion System involves the use of hollow, threaded cylinders that are implanted between two or more vertebrae. The implants are packed with bone graft to facilitate the growth of vertebral bone. Fusion is achieved when adjoining vertebrae grow together through and around the implants, resulting in stabilization.
Yet others have described apparatuses and/or methods intended for use in disc repair, though none appear to have been further developed at all, let alone to the point of commercialization. See, for instance, Garcia (French Patent Appl. No. FR 2 639 823) and Milner et al. (International Patent Appl. No. WO 9531948). Both references differ in several significant respects from each other and from the method described below. For instance, neither reference even contemplates, let alone addresses, the manner in which the amount of delivered material can or should be carefully controlled in order to achieve a desired pressure within the disc space. Nor does either reference contemplate or address the ability to shunt an initial portion of a curing biomaterial, in the course of delivering the biomaterial to the disc space.
Applicant has described prosthetic implants formed of biomaterials that can be delivered and cured in situ, e.g., using minimally invasive techniques. See for instance, applicant's U.S. Pat. No. 5,556,429 and published International Application WO 95/30388. Applicant's published International Application WO 97/26847 and International Application PCT/US97/20874 filed Nov. 14, 1997 (the disclosures of each of which are incorporated herein by reference) further describe, inter alia, the formation of a prosthetic nucleus within an intervertebral disc by a method that includes, for instance, the steps of inserting a collapsed mold apparatus (which in a preferred embodiment is described as a “balloon”) through a cannula that is itself positioned through an opening within the annulus, and filling the balloon with a flowable biomaterial that is adapted to cure in situ and provide a permanent disc replacement. See also, Applicant's “Porous Biomaterial and Biopolymer Resurfacing System” (PCT/US99/10004), as well as “Implantable Tissue Repair Device (PCT/US99/11740), and “Static Mixer” (PCT/US99/04407) applications.
In the course of further developing and evaluating the methods and systems described previously, it has become apparent that various further improvements are desired, e.g., in order to permit the doctor to determine and achieve a suitable intervertebral distraction pressure in the course of surgery, and in turn, to permit the doctor to controllably and effectively deliver a desired quantity of biomaterial to the balloon.